My 940 has been shuddering while idling. It alternates between smooth and shuddering. The RPMs stay stable, but when it shudders hard sometimes I think it wants to die. but it doesn't :P No check engine light.

Where should I begin ?

 

You probably have an engine misfire. Start by doing a basic tune up, make sure all vacuum lines are OK, clean the Idle Valve and the Throttle Body.

 

yeah, my first setup would be to pull and inspect the spark plugs after leaving it idling for 10 minutes or so (and then cooling sufficiently). also inspect the inside of the distributor cap...

odds are pretty that good new plugs (NGK, Volvo), good spark wires (bougicord), rotor & cap (bosch) will mostly cure this. if thats NOT it, then what lev says, check for vacuum and air intake path leaks, clean the idle valve & throttle body.

FWIW, my 1992 740T does a kind of single shudder every so often while idling when the AC kicks in. the RPMs dip but not long enough to show up on the tach. it never stalls, so I'm not really dug in.

 

Basic tune up was done, couldn't find any definitive vacuum leaks, IAC valve is clean and so is the throttle body now. Problem still persists, and now it is sluggish too.

Pulled codes and got:
2-2-3: Signal missing to/from idle valve
1-3-2: Battery voltage too high/low
1-4-3: Knock sensor signal absent or faulty

Sorry for late reply, I did not know people had responded to this until yesterday

 

hmm, check the harness connections to the idle valve and knock sensor.

perform "DTM #3" on the ECU (jumper position 2), one phase of this test will cycle the idle valve with the engine off (it will cycle through various actuators, includig the electric fan, idle controller, injectors, etc), so you can confirm its working or not. this test requires no tools or anything, its done with the same diagnostic box you read the codes from. for the idle test, I would consider disconnecting one of the air hoses that goes into the idle air valve, and attach a clean tube you can gently blow/suck on, and verify that when the DTM3 test cycles the IAC, you can ffeel the air pulsing.

see Engine and OBD Diagnostic Codes for the DTM#3 procedures.